sysutils/graylog: update to 4.0 and change license to SSPL

graylog 4.0 and higher now use a new license, the SSPL. For more
details, see https://www.graylog.org/post/graylog-v4-0-licensing-sspl

Submitted by: Charles Gonçalves <charles@freebsdbrasil.com.br>
Reviewed by: dch (maintainer)
Sponsored by: SkunkWerks, GmbH
Differential Revision: https://reviews.freebsd.org/D27253